The 'DANGER' MODEL: Last year we developed a new model of the immune system based on the idea that its primary function is to discriminate between dangerous and harmless things. We made several predictions and have begun this year to test some of them. 1) Neonatal tolerance: Female mice can be primed by male dendritic cells or tolerized by male B cells at any time of their life. This result was not predicted by the old 'self-non-self' model which proposed that tolerance to 'self' is se early in life. 2)B cell deficient mice: We predicted that B cells are not APCs for naive T cells and tested their importance in B cell deficient mice. The T cells in these animals are perfectly able to respond to protein antigens, minor H antigens and parasites. Their responses do not differ in any way from those of T cells in B cell sufficient mice. 3)B cells in adoptive transfer: Virgin F1 T cells transferred into parental mouse cannot use F1 B cells as APCs. We are now testing whether antigen presented by B cells is tolerogenic under these circumstances. 4)T cell help for killer: We predicted that CD8 killers receive CD4 help via the APC rather than directly from the helper cell. To test this, we have designed an in vitro model in which CD4 depleted CD8 T cell populations are unable to generate killers in vitro to the male antigen HY. We can reconstitute the activity by the addition of small numbers of cloned CD4 helpers but not with activated dendritic cells. We are treating the dendritic cells in various ways in attempts to overcome the CD4 deficiency. TOLERANCE 1) Maternal tolerance. Using quantitative PCR for the Y chromosome we occasionally find male cells in the organs of pregnant mice. This traffic is very sporadic and the cells are rejected by the maternal immune system while the fetus itself is not. Therefore, maternal acceptance of the fetus is most likely produced by the fetal- placental unit, rather than by fetal to maternal traffic. We predict, by the 'danger' model, that contact with the healthy fetal decidual cells should generate local tolerance in maternal lymphocytes. 2) Results of tolerance in Tg mice. To test which haplotypes support the development of a Tg TCR derived from an F1 mouse, we created T cell clones from F1 mice, isolated the genes for their antigen specific receptors and generated Tg mice. We now have four transgenics and are breeding them to SCID mice in order to analyse their receptors. T CELL MEMORY: We are studying killer cell memory in mice depleted of T helper cells and B cells. Thus far, killer cell memory lasts 9 months in the absence of T helper cells and 6 months in the absence of B cells. THE THYMIC STEM CELL is it precommitted?. We have found that the fetal thymus contains uncommitted stem cells at a frequency about ten fold fewer than fetal liver. Thus the commitment to T cell development occurs in the thymus, not earlier.